Modern clock housings are generally diecast from metal or injection molded from synthetic resin materials (plastics) and at least one movable wall affording access to the interior of the housing to allow the clockwork to be inserted and to afford access to the clockwork for repair or maintenance or to afford access to the hands of the clock for setting, adjustment or replacement.
The movable wall is generally either the back wall or the front wall or the bezel of the housing and, in the latter case, carries the window or glass through which the clock face is viewed. In some cases, both the front wall and the rear wall may have to be removable or detachable.
Generally speaking, the diecasting of metal housing and the injection molding of plastic housings are prone to fluctuations which cause considerable variation in wall thicknesses and in other dimensions of the fabricated housing. Conventional methods of assembling such housings with their movable walls have used snap-fitting formations formed directly on the wall and molded into the wall of the housing, or screw connections.
The interfitting formations are not practical where dimensional tolerances cannot be effectively controlled as is the case with both injection molding and diecasting, where the precision and reproducibility of the housing which is produced is a function of the conditions under which the shaping or matching operations take place. For example, both methods of manufacture are sensitive to the temperature of the die or mold and to variations of this temperature from point to point, fluctuations in the compositions and homogeneity of the materials which are used to fabricate the housing, factors such as the flow of the material and the like. As a consequence, the snap-fitting housings and movable walls in which the wall and the housing are both shaped with the interfitting formations directly, have not proved to be satisfactory.
The use of screw fastenings is not as sensitive to these variations and dimensions but involves other difficulties or problems. For example, in metal diecastings, it is necessary to cut screw threads or the like to accommodate the screws in costly operations.
With injection molded plastic housings, it is necessary to injection mold threaded metal bushings into the plastic material.
Even the use of self-tapping screws is associated with problems since such screws are generally relatively small and frequently are overdriven and stripped so that the housing becomes completely unusable.